The Headband of the Sage
is an intelligent item (71k gp) that gives you 4 skill ranks
in 6 (randomly set) knowledge skills. Unlike the Headband of Vast Intelligence
, these ranks do not depend on the wearer's HD.
If 2 of those 6 knowledge skills are Knowledge (Arcana)
and Knowledge (Religion)
, you can fulfill the skill rank prerequisites of the Mystic Theurge
prestige class right away at level 1.
With the Sunrod (or similar) trick, you can enter the Mystic Theurge class at level 3.
Should work for other prestige classes, that depend on knowledge skill ranks, as well. Though getting such an expensive item at low levels is probably hard.
Edit: I think it's also possible to enter Mystic Theurge at level 3 with an ordinary Headband of Vast Intelligence +4
. The usual "you have to wait until level 4" is because you cannot put ranks in skills before you take a new class when getting a new level. However, the Headband grants skill ranks equal to a character's Hit Dice, and unlike rolling for hit points during level-up after taking a class, your HD should advance immediately before taking a class.